Toward Hyperpolarized 19 F Molecular Imaging via Reversible Exchange with Parahydrogen

Enhancing the NMR signals of plant oil components using hyperpolarisation relayed via proton exchange

In this work, the limited sensitivity of magnetic resonance is addressed by using the hyperpolarisation method relayed signal amplification by reversible exchange (SABRE-Relay) to transfer latent magnetism from para-hydrogen, a readily isolated spin isomer of hydrogen gas, to components of key plant oils such as citronellol, geraniol, and nerol. This is achieved via relayed polarisation transfer in which an [Ir(H)2(IMes)(NH2R)3]Cl type complex produces hyperpolarised NH2R free in solution, before labile proton exchange between the hyperpolarisation carrier (NH2R) and the OH-containing plant oil component generates enhanced NMR signals for the latter. Consequently, up to ca. 200-fold 1H (0.65% 1H polarisation) and 800-fold 13C NMR signal enhancements (0.65% 13C polarisation) are recorded for these essential oils in seconds. Remarkably, the resulting NMR signals are not only diagnostic, but prove to propagate over large spin systems via a suitable coupling network. A route to optimise the enhancement process by varying the identity of the carrier NH2R, and its concentration is demonstrated. In order to prove utility, these pilot measurements are extended to study a much wider range of plant-derived molecules including rhodinol, verbenol, (1R)-endo-(+)-fenchyl alcohol, (-)-carveol, and linalool. Further measurements are then described which demonstrate citronellol and geraniol can be detected in an off-the-shelf healthcare product rose geranium oil at concentrations of just a few tens of μM in single scan 1H NMR measurements, which are not visible in comparable thermally polarised NMR experiments. This work therefore presents a significant expansion of the types of molecules amenable to hyperpolarisation using para-hydrogen and illustrates a real-world application in the diagnostic detection of low concentration analytes in mixtures.

Enhancing 19F Benchtop NMR Spectroscopy by Combining para-Hydrogen Hyperpolarization and Multiplet Refocusing

Benchtop NMR spectrometers provide a promising alternative to high-field NMR for applications that are limited by instrument size and/or cost. ¹⁹F benchtop NMR is attractive due to the larger chemical shift range of ¹⁹F relative to ¹H and the lack of background signal in most applications. However, practical applications of benchtop ¹⁹F NMR are limited by its low sensitivity due to the relatively weak field strengths of benchtop NMR spectrometers. Here we present a sensitivity-enhancement strategy that combines SABRE (Signal Amplification By Reversible Exchange) hyperpolarization with the multiplet refocusing method SHARPER (Sensitive, Homogeneous, And Resolved PEaks in Real time). When applied to a range of fluoropyridines, SABRE-SHARPER achieves overall signal enhancements of up to 5700-fold through the combined effects of hyperpolarization and line-narrowing. This approach can be generalized to the analysis of mixtures through the use of a selective variant of the SHARPER sequence, selSHARPER. The ability of SABRE-selSHARPER to simultaneously boost sensitivity and discriminate between two components of a mixture is demonstrated, where selectivity is achieved through a combination of selective excitation and the choice of polarization transfer field during the SABRE step.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Rapid 13C Hyperpolarization of the TCA Cycle Intermediate α-Ketoglutarate via SABRE-SHEATH

α-Ketoglutarate is a key biomolecule involved in a number of metabolic pathways─most notably the TCA cycle. Abnormal α-ketoglutarate metabolism has also been linked with cancer. Here, isotopic labeling was employed to synthesize [1-13C,5-12C,D4]α-ketoglutarate with the future goal of utilizing its [1-13C]-hyperpolarized state for real-time metabolic imaging of α-ketoglutarate analytes and its downstream metabolites in vivo. The signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH) hyperpolarization technique was used to create 9.7% [1-13C] polarization in 1 minute in this isotopologue. The efficient 13C hyperpolarization, which utilizes parahydrogen as the source of nuclear spin order, is also supported by favorable relaxation dynamics at 0.4 μT field (the optimal polarization transfer field): the exponential 13C polarization buildup constant Tb is 11.0 ± 0.4 s whereas the 13C polarization decay constant T1 is 18.5 ± 0.7 s. An even higher 13C polarization value of 17.3% was achieved using natural-abundance α-ketoglutarate disodium salt, with overall similar relaxation dynamics at 0.4 μT field, indicating that substrate deuteration leads only to a slight increase (∼1.2-fold) in the relaxation rates for 13C nuclei separated by three chemical bonds. Instead, the gain in polarization (natural abundance versus [1-13C]-labeled) is rationalized through the smaller heat capacity of the "spin bath" comprising available 13C spins that must be hyperpolarized by the same number of parahydrogen present in each sample, in line with previous 15N SABRE-SHEATH studies. Remarkably, the C-2 carbon was not hyperpolarized in both α-ketoglutarate isotopologues studied; this observation is in sharp contrast with previously reported SABRE-SHEATH pyruvate studies, indicating that the catalyst-binding dynamics of C-2 in α-ketoglutarate differ from that in pyruvate. We also demonstrate that 13C spectroscopic characterization of α-ketoglutarate and pyruvate analytes can be performed at natural 13C abundance with an estimated detection limit of 80 micromolar concentration × *%P13C. All in all, the fundamental studies reported here enable a wide range of research communities with a new hyperpolarized contrast agent potentially useful for metabolic imaging of brain function, cancer, and other metabolically challenging diseases.

Advancing homogeneous catalysis for parahydrogen-derived hyperpolarisation and its NMR applications

Parahydrogen-induced polarisation (PHIP) is a nuclear spin hyperpolarisation technique employed to enhance NMR signals for a wide range of molecules. This is achieved by exploiting the chemical reactions of parahydrogen (para-H2), the spin-0 isomer of H2. These reactions break the molecular symmetry of para-H2 in a way that can produce dramatically enhanced NMR signals for reaction products, and are usually catalysed by a transition metal complex. In this review, we discuss recent advances in novel homogeneous catalysts that can produce hyperpolarised products upon reaction with para-H2. We also discuss hyperpolarisation attained in reversible reactions (termed signal amplification by reversible exchange, SABRE) and focus on catalyst developments in recent years that have allowed hyperpolarisation of a wider range of target molecules. In particular, recent examples of novel ruthenium catalysts for trans and geminal hydrogenation, metal-free catalysts, iridium sulfoxide-containing SABRE systems, and cobalt complexes for PHIP and SABRE are reviewed. Advances in this catalysis have expanded the types of molecules amenable to hyperpolarisation using PHIP and SABRE, and their applications in NMR reaction monitoring, mechanistic elucidation, biomedical imaging, and many other areas, are increasing.

Hyperpolarised NMR to aid molecular profiling of electronic cigarette aerosols

Signal amplification by reversible exchange (SABRE) hyperpolarisation is used to enhance the NMR signals of nicotine and acrolein in methanol-d4 solutions of electronic cigarette aerosols. Consequently, detection of 74 μM nicotine is possible in just a single scan 1H NMR spectrum. The first example of an aldehyde hyperpolarised using SABRE is demonstrated and we work towards novel real-world applications of SABRE-hyperpolarised NMR for chemical analysis.

A Versatile Compact Parahydrogen Membrane Reactor

We introduce a Spin Transfer Automated Reactor (STAR) that produces continuous parahydrogen induced polarization (PHIP), which is stable for hours to days. We use the PHIP variant called signal amplification by reversible exchange (SABRE), which is particularly well suited to produce continuous hyperpolarization. The STAR is operated in conjunction with benchtop (1.1 T) and high field (9.4 T) NMR magnets, highlighting the versatility of this system to operate with any NMR or MRI system. The STAR uses semipermeable membranes to efficiently deliver parahydrogen into solutions at nano to milli Tesla fields, which enables 1 H, 13 C, and 15 N hyperpolarization on a large range of substrates including drugs and metabolites. The unique features of the STAR are leveraged for important applications, including continuous hyperpolarization of metabolites, desirable for examining steady-state metabolism in vivo, as well as for continuous RASER signals suitable for the investigation of new physics.

15 N hyperpolarisation of the antiprotozoal drug ornidazole by Signal Amplification By Reversible Exchange in aqueous medium

Signal amplification by reversible exchange (SABRE) offers a cost-effective route to boost nuclear magnetic resonance (NMR) signal by several orders of magnitude by employing readily available para-hydrogen as a source of hyperpolarisation. Although ¹ H spins have been the natural choice of SABRE hyperpolarisation since its inception due to its simplicity and accessibility, limited spin lifetimes of ¹ H makes it harder to employ them in a range of time-dependent NMR experiments. Heteronuclear spins, for example, ¹³ C and ¹⁵ N, in general have much longer T₁ lifetimes and thereby are found to be more suitable for hyperpolarised biological applications as demonstrated previously by para-hydrogen induced polarisation (PHIP) and dynamic nuclear polarisation (DNP). In this study we demonstrate a simple procedure to enhance ¹⁵ N signal of an antibiotic drug ornidazole by up to 71,000-folds with net ¹⁵ N polarisation reaching ~23%. Further, the effect of co-ligand strategy is studied in conjunction with the optimum field transfer protocols and consequently achieving ¹⁵ N hyperpolarised spin lifetime of >3 min at low field. Finally, we present a convenient route to harness the hyperpolarised solution in aqueous medium free from catalyst contamination leading to a strong ¹⁵ N signal detection for an extended duration of time.

Singlet to triplet conversion in molecular hydrogen and its role in parahydrogen induced polarization

Detailed experimental and comprehensive theoretical analysis of singlet-triplet conversion in molecular hydrogen dissolved in a solution together with organometallic complexes used in experiments with parahydrogen (the H₂ molecule in its nuclear singlet spin state) is reported. We demonstrate that this conversion, which gives rise to formation of orthohydrogen (the H₂ molecule in its nuclear triplet spin state), is a remarkably efficient process that strongly reduces the resulting NMR (nuclear magnetic resonance) signal enhancement, here of ¹⁵N nuclei polarized at high fields using suitable NMR pulse sequences. We make use of a simple improvement of traditional pulse sequences, utilizing a single pulse on the proton channel that gives rise to an additional strong increase of the signal. Furthermore, analysis of the enhancement as a function of the pulse length allows one to estimate the actual population of the spin states of H₂. We are also able to demonstrate that the spin conversion process in H₂ is strongly affected by the concentration of ¹⁵N nuclei. This observation allows us to explain the dependence of the ¹⁵N signal enhancement on the abundance of ¹⁵N isotopes.

Low-Cost High-Pressure Clinical-Scale 50% Parahydrogen Generator Using Liquid Nitrogen at 77 K

We report on a robust and low-cost parahydrogen generator design employing liquid nitrogen as a coolant. The core of the generator consists of catalyst-filled spiral copper tubing, which can be pressurized to 35 atm. Parahydrogen fraction >48% was obtained at 77 K with three nearly identical generators using paramagnetic hydrated iron oxide catalysts. Parahydrogen quantification was performed on the fly via benchtop NMR spectroscopy to monitor the signal from residual orthohydrogen-parahydrogen is NMR silent. This real-time quantification approach was also used to evaluate catalyst activation at up to 1.0 standard liter per minute flow rate. The reported inexpensive device can be employed for a wide range of studies employing parahydrogen as a source of nuclear spin hyperpolarization. To this end, we demonstrate the utility of this parahydrogen generator for hyperpolarization of concentrated sodium [1-13C]pyruvate, a metabolic contrast agent under investigation in numerous clinical trials. The reported pilot optimization of SABRE-SHEATH (signal amplification by reversible exchange-shield enables alignment transfer to heteronuclei) hyperpolarization yielded 13C signal enhancement of over 14,000-fold at a clinically relevant magnetic field of 1 T corresponding to approximately 1.2% 13C polarization-if near 100% parahydrogen would have been employed, the reported value would be tripled to 13C polarization of 3.5%.

Synthesis and 15 N NMR Signal Amplification by Reversible Exchange of [15 N]Dalfampridine at Microtesla Magnetic Fields

Signal Amplification by Reversible Exchange (SABRE) technique enables nuclear spin hyperpolarization of wide range of compounds using parahydrogen. Here we present the synthetic approach to prepare ¹⁵ N-labeled [¹⁵ N]dalfampridine (4-amino[¹⁵ N]pyridine) utilized as a drug to reduce the symptoms of multiple sclerosis. The synthesized compound was hyperpolarized using SABRE at microtesla magnetic fields (SABRE-SHEATH technique) with up to 2.0 % ¹⁵ N polarization. The 7-hour-long activation of SABRE pre-catalyst [Ir(IMes)(COD)Cl] in the presence of [¹⁵ N]dalfampridine can be remedied by the use of pyridine co-ligand for catalyst activation while retaining the ¹⁵ N polarization levels of [¹⁵ N]dalfampridine. The effects of experimental conditions such as polarization transfer magnetic field, temperature, concentration, parahydrogen flow rate and pressure on ¹⁵ N polarization levels of free and equatorial catalyst-bound [¹⁵ N]dalfampridine were investigated. Moreover, we studied ¹⁵ N polarization build-up and decay at magnetic field of less than 0.04 μT as well as ¹⁵ N polarization decay at the Earth's magnetic field and at 1.4 T.

Reversible Hyperpolarization of Ketoisocaproate Using Sulfoxide-containing Polarization Transfer Catalysts

The substrate scope of sulfoxide-containing magnetisation transfer catalysts is extended to hyperpolarize α-ketoisocaproate and α-ketoisocaproate-1-[13 C]. This is achieved by forming [Ir(H)2 (κ2 -ketoisocaproate)(N-heterocyclic carbene)(sulfoxide)] which transfers latent magnetism from p-H2 via the signal amplification by reversible exchange (SABRE) process. The effect of polarization transfer field on the formation of enhanced 13 C magnetization is evaluated. Consequently, performing SABRE in a 0.5 μT field enabled most efficient magnetisation transfer. 13 C NMR signals for α-ketoisocaproate-1-[13 C] in methanol-d4 are up to 985-fold more intense than their traditional Boltzmann derived signal intensity (0.8 % 13 C polarisation). Single crystal X-ray diffraction reveals the formation of the novel catalyst decomposition products [Ir(μ-H)(H)2 (IMes)(SO(Ph)(Me)2 )]2 and [(Ir(H)2 (IMes)(SO(Me)2 ))2 (μ-S)] when the sulfoxides methylphenylsulfoxide and dimethylsulfoxide are used respectively.

Benchtop NMR analysis of piperazine-based drugs hyperpolarised by SABRE

Piperazine-based drugs, such as N-benzylpiperazine (BZP), became attractive in the 2000s due to possessing effects similar to amphetamines. Herein, BZP, in addition to its pyridyl analogues, 2-, 3-, and 4-pyridylmethylpiperidine (2-PMP, 3-PMP, and 4-PMP respectively) was subjected to the hyperpolarisation technique Signal Amplification By Reversible Exchange (SABRE) in order to demonstrate the use of this technique to detect these piperazine-based drugs. Although BZP was not hyperpolarised via SABRE, 2-PMP, 3-PMP, and 4-PMP were, with the ortho- and meta-pyridyl protons of 4-PMP showing the largest enhancement of 313-fold and 267-fold, respectively, in a 1.4-T detection field, following polarisation transfer at Earth's magnetic field. In addition to the freebase, 4-PMP.3HCl was also appraised by SABRE and was found not to polarise, however, the addition of increasing equivalents of triethylamine (TEA) produced the freebase, with a maximum enhancement observed upon the addition of 3 equivalents of TEA. Further addition of TEA led to a reduction in the observed enhancement. SABRE was also employed to polarise 4-PMP.3HCl (~20% w/w) in a simulated tablet to demonstrate the forensic application of the technique (138-fold enhancement for the ortho-pyridyl protons). The amount of 4-PMP.3HCl present in the simulated tablet was quantified via NMR using D₂ O as a solvent and compared well to complimentary gas chromatography-mass spectrometry data. Exchanging D₂ O for CD₃ OD as the solvent utilised for analysis resulted in a significantly lower amount of 4-PMP.3HCl being determined, thus highlighting safeguarding issues linked to drug abuse in relation to determining the amount of active pharmaceutical ingredient present.

Theoretical description of hyperpolarization formation in the SABRE-relay method

SABRE (Signal Amplification By Reversible Exchange) has become a widely used method for hyper-polarizing nuclear spins, thereby enhancing their Nuclear Magnetic Resonance (NMR) signals by orders of magnitude. In SABRE experiments, the non-equilibrium spin order is transferred from parahydrogen to a substrate in a transient organometallic complex. The applicability of SABRE is expanded by the methodology of SABRE-relay in which polarization can be relayed to a second substrate either by direct chemical exchange of hyperpolarized nuclei or by polarization transfer between two substrates in a second organometallic complex. To understand the mechanism of the polarization transfer and study the transfer efficiency, we propose a theoretical approach to SABRE-relay, which can treat both spin dynamics and chemical kinetics as well as the interplay between them. The approach is based on a set of equations for the spin density matrices of the spin systems involved (i.e., SABRE substrates and complexes), which can be solved numerically. Using this method, we perform a detailed study of polarization formation and analyze in detail the dependence of the attainable polarization level on various chemical kinetic and spin dynamic parameters. We foresee the applications of the present approach for optimizing SABRE-relay experiments with the ultimate goal of achieving maximal NMR signal enhancements for substrates of interest.

Rational ligand choice extends the SABRE substrate scope

Here we report on chelating ligands for Signal Amplification By Reversible Exchange (SABRE) catalysts that permit hyperpolarisation on otherwise sterically hindered substrates. We demonstrate 1H enhancements of ∼100-fold over 8.5 T thermal for 2-substituted pyridines, and smaller, yet significant enhancements for provitamin B6 and caffeine. We also show 15N-enhancements of ∼1000-fold and 19F-enhancements of 30-fold.

Quantifying the effects of quadrupolar sinks via15N relaxation dynamics in metronidazoles hyperpolarized via SABRE-SHEATH

15N spin-lattice relaxation dynamics in metronidazole-15N3 and metronidazole-15N2 isotopologues are studied for rational design of 15N-enriched biomolecules for signal amplification by reversible exchange in microtesla fields. 15N relaxation dynamics mapping reveals the deleterious effects of interactions with the polarization transfer catalyst and a quadrupolar 14N nucleus within the spin-relayed 15N-15N network.

SABRE polarized low field rare-spin spectroscopy

High-field nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for identification and characterization of chemicals and biomolecular structures. In the vast majority of NMR experiments, nuclear spin polarization arises from thermalization in multi-Tesla magnetic fields produced by superconducting magnets. In contrast, NMR instruments operating at low magnetic fields are emerging as a compact, inexpensive, and highly accessible alternative but suffer from low thermal polarization at a low field strength and consequently a low signal. However, certain hyperpolarization techniques create high polarization levels on target molecules independent of magnetic fields, giving low-field NMR a significant sensitivity boost. In this study, SABRE (Signal Amplification By Reversible Exchange) was combined with high homogeneity electromagnets operating at mT fields, enabling high resolution ¹H, ¹³C, ¹⁵N, and ¹⁹F spectra to be detected with a single scan at magnetic fields between 1 mT and 10 mT. Chemical specificity is attained at mT magnetic fields with complex, highly resolved spectra. Most spectra are in the strong coupling regime where J-couplings are on the order of chemical shift differences. The spectra and the hyperpolarization spin dynamics are simulated with SPINACH. The simulations start from the parahydrogen singlet in the bound complex and include both chemical exchange and spin evolution at these mT fields. The simulations qualitatively match the experimental spectra and are used to identify the spin order terms formed during mT SABRE. The combination of low field NMR instruments with SABRE polarization results in sensitive measurements, even for rare spins with low gyromagnetic ratios at low magnetic fields.

"Direct" 13 C Hyperpolarization of 13 C-Acetate by MicroTesla NMR Signal Amplification by Reversible Exchange (SABRE)

Herein, we demonstrate "direct" ¹³ C hyperpolarization of ¹³ C-acetate via signal amplification by reversible exchange (SABRE). The standard SABRE homogeneous catalyst [Ir-IMes; [IrCl(COD)(IMes)], (IMes=1,3-bis(2,4,6-trimethylphenyl), imidazole-2-ylidene; COD=cyclooctadiene)] was first activated in the presence of an auxiliary substrate (pyridine) in alcohol. Following addition of sodium 1-¹³ C-acetate, parahydrogen bubbling within a microtesla magnetic field (i.e. under conditions of SABRE in shield enables alignment transfer to heteronuclei, SABRE-SHEATH) resulted in positive enhancements of up to ≈100-fold in the ¹³ C NMR signal compared to thermal equilibrium at 9.4 T. The present results are consistent with a mechanism of "direct" transfer of spin order from parahydrogen to ¹³ C spins of acetate weakly bound to the catalyst, under conditions of fast exchange with respect to the ¹³ C acetate resonance, but we find that relaxation dynamics at microtesla fields alter the optimal matching from the traditional SABRE-SHEATH picture. Further development of this approach could lead to new ways to rapidly, cheaply, and simply hyperpolarize a broad range of substrates (e.g. metabolites with carboxyl groups) for various applications, including biomedical NMR and MRI of cellular and in vivo metabolism.

From LASER physics to the para-hydrogen pumped RASER

The properties of the LASER with respect to self-organization are compared with the key features of the p-H₂ pumped RASER. According to LASER theory the equations of motion for the LASER can be derived from the enslaving principle, i.e. the slowest-changing order parameter (the light field in the resonator) enslaves the rapidly relaxing atomic degrees of freedom. Likewise, it is shown here that the equations of motion for the p-H₂ pumped RASER result from a set of order parameters, where the transverse magnetization of the RASER-active spin states enslaves the electromagnetic modes. The consequences are striking for nuclear magnetic resonance (NMR) spectroscopy, since long-lasting multi-mode RASER oscillations enable unprecedented spectroscopic resolution down to the micro-Hertz regime. Based on the theory for multi-mode RASER operation we analyze the conditions that reveal either the collapse of the entire NMR spectrum, the occurrence of self-organized frequency-combs, or RASER spectra which reflect the J-coupled network of the molecule. Certain RASER experiments involving the protons of ¹⁵N pyridine or 3-picoline molecules pumped with p-H₂ via SABRE (Signal Amplification By Reversible Exchange) show either a single RASER oscillation in the time domain, giant RASER pulses or a complex RASER beat pattern. The corresponding ¹H spectra consist of one narrow line, equidistant narrow lines (frequency-comb), or highly resolved lines reporting NMR properties, respectively. Numerous applications in the areas of material sciences, fundamental physics and medicine involving high precision sensors for magnetic fields, rotational motions or molecular structures become feasible.

Low-Concentration Measurements of Nuclear Spin-Induced Optical Rotation Using SABRE Hyperpolarization

Nuclear spin-induced optical rotation (NSOR) is a promising phenomenon for molecular structure elucidation due to its sensitivity to electronic structure near atomic nuclei. It is the only experimentally verified nuclear magneto-optic effect (NMOE), so far observed usually in neat liquids or in concentrated binary mixtures, with the proportion of the minor component at least 10%. We report a method to extend the lower-concentration range of NSOR measurements by 2 orders of magnitude by employing continuous-flow SABRE (signal amplification by reversible exchange) hyperpolarization. This approach significantly increases the sensitivity of NSOR and enables its detection in dilute samples, as demonstrated with measurements of NSOR of 90 mmol/L solutions of pyridine and pyrazine. The results are compared with first-principles calculations, and good agreement is found. The possibility to measure low-concentration solutions significantly extends the pool of samples available for further studies of NMOEs.

Amino Acid-Derived Sensors for Specific Zn2+ Detection Using Hyperpolarized 13 C Magnetic Resonance Spectroscopy

Alterations in Zn²⁺ concentration are seen in normal tissues and in disease states, and for this reason imaging of Zn²⁺ is an area of active investigation. Herein, enriched [1-¹³ C]cysteine and [1-¹³ C₂ ]iminodiacetic acid were developed as Zn²⁺ -specific imaging probes using hyperpolarized ¹³ C magnetic resonance spectroscopy. [1-¹³ C]cysteine was used to accurately quantify Zn²⁺ in complex biological mixtures. These sensors can be employed to detect Zn²⁺ via imaging mechanisms including changes in ¹³ C chemical shift, resonance linewidth, or T₁ .

Quasi-Resonance Fluorine-19 Signal Amplification by Reversible Exchange

We report on an extension of the quasi-resonance (QUASR) pulse sequence used for signal amplification by reversible exchange (SABRE), showing that we may target distantly J-coupled 19F-spins. Polarization transfer from the parahydrogen-derived hydrides to the 19F nucleus is accomplished via weak five-bond J-couplings using a shaped QUASR radio frequency pulse at a 0.05 T magnetic field. The net result is the direct generation of hyperpolarized 19F z-magnetization, derived from the parahydrogen singlet order. An accumulation of 19F polarization on the free ligand is achieved with subsequent repetition of this pulse sequence. The hyperpolarized 19F signal exhibits clear dependence on the pulse length, irradiation frequency, and delay time in a manner similar to that reported for 15N QUASR-SABRE. Moreover, the hyperpolarized 19F signals of 3-19F-14N-pyridine and 3-19F-15N-pyridine isotopologues are similar, suggesting that (i) polarization transfer via QUASR-SABRE is irrespective of the nitrogen isotopologue and (ii) the presence or absence of the spin-1/2 15N nucleus has no impact on the efficiency of QUASR-SABRE polarization transfer. Although optimization of polarization transfer efficiency to 19F (P19F ≈ 0.1%) was not the goal of this study, we show that high-field SABRE can be efficient and broadly applicable for direct hyperpolarization of 19F spins.

15 N MRI of SLIC-SABRE Hyperpolarized 15 N-Labelled Pyridine and Nicotinamide

Magnetic Resonance Imaging (MRI) is a powerful non-invasive diagnostic method extensively used in biomedical studies. A significant limitation of MRI is its relatively low signal-to-noise ratio, which can be increased by hyperpolarizing nuclear spins. One promising method is Signal Amplification By Reversible Exchange (SABRE), which employs parahydrogen as a source of hyperpolarization. Recent studies demonstrated the feasibility to improve MRI sensitivity with this hyperpolarization technique. Hyperpolarized 15 N nuclei in biomolecules can potentially retain their spin alignment for tens of minutes, providing an extended time window for the utilization of the hyperpolarized compounds. In this work, we demonstrate for the first time that radio-frequency-based SABRE hyperpolarization techniques can be used to obtain 15 N MRI of biomolecule 1-15 N-nicotinamide. Two image acquisition strategies were utilized and compared: Single Point Imaging (SPI) and Fast Low Angle SHot (FLASH). These methods demonstrated opportunities of high-field SABRE for biomedical applications.

Hyperpolarised 1H–13C Benchtop NMR Spectroscopy

Benchtop NMR spectrometers with sub-ppm spectral resolution have opened up new opportunities for performing NMR outside of the standard laboratory environment. However, the relatively weak magnetic fields of these devices (1–2 T) results in low sensitivity and significant peak overlap in 1H NMR spectra. Here, we use hyperpolarised 13C{1H} NMR to overcome these challenges. Specifically, we demonstrate the use of the signal amplification by reversible exchange (SABRE) parahydrogen-based hyperpolarisation technique to enhance the sensitivity of natural abundance 1D and 2D 13C{1H} benchtop NMR spectra. We compare two detection methods for SABRE-enhanced 13C NMR and observe an optimal 13C{1H} signal-to-noise ratio (SNR) for a refocused INEPT approach, where hyperpolarisation is transferred from 1H to 13C. In addition, we exemplify SABRE-enhanced 2D 13C benchtop NMR through the acquisition of a 2D HETCOR spectrum of 260 mM of 4-methylpyridine at natural isotopic abundance in a total experiment time of 69 min. In theory, signal averaging for over 300 days would be required to achieve a comparable SNR for a thermally polarised benchtop NMR spectrum acquired of a sample of the same concentration at natural abundance.

High-throughput continuous-flow system for SABRE hyperpolarization

Signal Amplification By Reversible Exchange (SABRE) is a versatile method for hyperpolarizing small organic molecules that helps to overcome the inherent low signal-to-noise ratio of nuclear magnetic resonance (NMR) measurements. It offers orders of magnitude enhanced signal strength, but the obtained nuclear polarization usually rapidly relaxes, requiring a quick transport of the sample to the spectrometer. Here we report a new design of a polarizing system, which can be used to prepare a continuous flow of SABRE-hyperpolarized sample with a considerable throughput of several millilitres per second and a rapid delivery into an NMR instrument. The polarizer performance under different conditions such as flow rate of the hydrogen or liquid sample is tested by measuring a series of NMR spectra and magnetic resonance images (MRI) of hyperpolarized pyridine in methanol. Results show a capability to continuously produce sample with dramatically enhanced signal over two orders of magnitude. The constant supply of hyperpolarized sample can be exploited, e.g., in experiments requiring multiple repetitions, such as 2D- and 3D-NMR or MRI measurements, and also naturally allows measurements of flow maps, including systems with high flow rates, for which the level of achievable thermal polarization might not be usable any more. In addition, the experiments can be viably carried out in a non-deuterated solvent, due to the effective suppression of the thermal polarization by the fast sample flow. The presented system opens the possibilities for SABRE experiments requiring a long-term, stable and high level of nuclear polarization.

A versatile synthetic route to the preparation of 15 N heterocycles

A robust medium-scale (approximately 3 g) synthetic method for ¹⁵ N labeling of pyridine (¹⁵ N-Py) is reported based on the Zincke reaction. ¹⁵ N enrichment in excess of 81% was achieved with approximately 33% yield. ¹⁵ N-Py serves as a standard substrate in a wide range of studies employing a hyperpolarization technique for efficient polarization transfer from parahydrogen to heteronuclei; this technique, called SABRE (signal amplification by reversible exchange), employs a simultaneous chemical exchange of parahydrogen and a to-be-hyperpolarized substrate (e.g., pyridine) on metal centers. In studies aimed at the development of hyperpolarized contrast agents for in vivo molecular imaging, pyridine is often employed either as a model substrate (for hyperpolarization technique development, quality assurance, and phantom imaging studies) or as a co-substrate to facilitate more efficient hyperpolarization of a wide range of emerging contrast agents (e.g., nicotinamide). Here, the produced ¹⁵ N-Py was used for the feasibility study of spontaneous ¹⁵ N hyperpolarization at high magnetic (HF) fields (7 T and 9.4 T) of an NMR spectrometer and an MRI scanner. SABRE hyperpolarization enabled acquisition of 2D MRI imaging of catalyst-bound ¹⁵ N-pyridine with 75 × 75 mm² field of view (FOV), 32 × 32 matrix size, demonstrating the feasibility of ¹⁵ N HF-SABRE molecular imaging with 2.4 × 2.4 mm² spatial resolution.

Parahydrogen induced hyperpolarization provides a tool for NMR metabolomics at nanomolar concentrations

Sensitivity enhancement by parahydrogen hyperpolarization allows NMR detection and quantification of hundreds of urinary metabolites at down to nanomolar concentrations.

Only Para-Hydrogen Spectroscopy (OPSY) Revisited: In-Phase Spectra for Chemical Analysis and Imaging

We revisited only para-hydrogen spectroscopy (OPSY) for the analysis of para-hydrogen-enhanced NMR spectra at high magnetic fields. We found that the sign of the gradients and interpulse delays are pivotal for the performance of the sequence: the variant of double-quantum filter OPSY, where the second time interval is twice as long as the first one (OPSYd-12) converts the antiphase spectrum to in-phase and efficiently suppresses the background signal in a single scan better than the other variants. OPSYd-12 strongly facilitates the analysis of para-hydrogen-derived NMR spectra in homogeneous and inhomogeneous magnetic fields. Furthermore, the net magnetization produced is essential for subsequent applications such as imaging, e.g., in a reaction chamber or in vivo.

Chemical Exchange Reaction Effect on Polarization Transfer Efficiency in SLIC-SABRE

Signal Amplification By Reversible Exchange (SABRE) is a new and rapidly developing hyperpolarization technique. The recent discovery of Spin-Lock Induced Crossing SABRE (SLIC-SABRE) showed that high field hyperpolarization transfer techniques developed so far were optimized for singlet spin order that does not coincide with the experimentally produced spin state. Here, we investigated the SLIC-SABRE approach and the most advanced quantitative theoretical SABRE model to date. Our goal is to achieve the highest possible polarization with SLIC-SABRE at high field using the standard SABRE system, IrIMes catalyst with pyridine. We demonstrated the accuracy of the SABRE model describing the effects of various physical parameters such as the amplitude and frequency of the radio frequency field, and the effects of chemical parameters such as the exchange rate constants. By fitting the model to the experimental data, the effective life time of the SABRE complex was estimated, as well as the entropy and enthalpy of the complex-dissociation reaction. We show, for the first time, that this SLIC-SABRE model can be useful for the evaluation of the chemical exchange parameters that are very important for the production of highly polarized contrast agents via SABRE.

19F Hyperpolarization of 15N-3-19F-Pyridine Via Signal Amplification by Reversible Exchange

We report synthesis of ¹⁵N-3-¹⁹F-pyridine via Zincke salt formation with the overall 35% yield and 84% ¹⁵N isotopic purity. Hyperpolarization studies of Signal Amplification by Reversible Exchange (SABRE) and SABRE in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) were performed to investigate the mechanism of polarization transfer from parahydrogen-derived hydride protons to ¹⁹F nucleus in milli-Tesla and micro-Tesla magnetic field regimes in ¹⁵N-3-¹⁹F-pyridine and ¹⁴N-3-¹⁹F-pyridine. We found the mismatch between ¹⁵N and ¹⁹F magnetic field hyperpolarization profiles in the micro-Tesla regime indicating that the spontaneous hyperpolarization process likely happens directly from parahydrogen-derived hydride protons to ¹⁹F nucleus without spin-relaying via ¹⁵N site. In case of SABRE magnetic field regime (milli-Tesla magnetic field range), we found that magnetic field profiles for ¹H and ¹⁹F hyperpolarization are very similar, and ¹⁹F polarization levels are significantly lower than ¹H SABRE polarization levels and lower than ¹⁹F SABRE-SHEATH (i.e. obtained at micro-Tesla magnetic field) polarization levels. Our findings support the hypothesis that in milli-Tesla magnetic field regime, the process of ¹⁹F nuclei hyperpolarization is relayed via protons of substrate, and therefore is very inefficient. These findings are important in the context of improvement of the hyperpolarization hardware and rational design of the hyperpolarized molecular probes.

Harnessing asymmetric N-heterocyclic carbene ligands to optimise SABRE hyperpolarisation

The catalytic signal amplification by reversible exchange process has become widely used for the hyperpolarisation of small molecules to improve their magnetic resonance detectability. It harnesses the latent polarisation of parahydrogen, and involves the formation of a labile metal complex that often contains an N-heterocyclic carbene (NHC) ligand (e.g. [Ir(H)2(NHC)(pyridine)3]Cl), which act as a polarisation transfer catalyst. Unfortunately, if the target molecule is too bulky, binding to the catalyst is poor and the hyperpolarisation yield is therefore low. We illustrate here the behaviour of a series of asymmetric NHC containing catalysts towards 3,4- and 3,5-lutidine in order to show how catalyst design can be used to dramatically improve the outcome of this catalytic process for sterically encumbered ligands.

Parahydrogen-Based Hyperpolarization for Biomedicine

Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2 ), which is inherently stable and long-lived. When brought into contact with another molecule, this "spin order on demand" allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2 -based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.

Facile Removal of Homogeneous SABRE Catalysts for Purifying Hyperpolarized Metronidazole, a Potential Hypoxia Sensor

We report a simple and effective method to remove IrIMes homogeneous polarization transfer catalysts from solutions where NMR Signal Amplification By Reversible Exchange (SABRE) has been performed, while leaving intact the substrate's hyperpolarized state. Following microTesla SABRE hyperpolarization of ¹⁵N spins in metronidazole, addition of SiO₂ microparticles functionalized with 3-mercaptopropyl or 2-mercaptoethyl ethyl sulfide moieties provides removal of the catalyst from solution well within the hyperpolarization decay time at 0.3 T (T ₁>3 mins)-and enabling transfer to 9.4 T for detection of enhanced ¹⁵N signals in the absence of catalyst within the NMR-detection region. Successful catalyst removal from solution is supported by the inability to "re-hyperpolarize" ¹⁵N spins in subsequent attempts, as well as by ¹H NMR and ICP-MS. Record-high ¹⁵N nuclear polarization of up to ~34% was achieved, corresponding to >100,000-fold enhancement at 9.4 T, and approximately 5/6^th of the ¹⁵N hyperpolarization is retained after ~20-second-long purification procedure. Taken together, these results help pave the way for future studies involving in vivo molecular imaging using agents hyperpolarized via rapid and inexpensive parahydrogen-based methods.

Toward Cleavable Metabolic/pH Sensing "Double Agents" Hyperpolarized by NMR Signal Amplification by Reversible Exchange

We show the simultaneous generation of hyperpolarized 13 C-labeled acetate and 15 N-labeled imidazole following spin-relay of hyperpolarization and hydrolysis of the acetyl moiety on 1-13 C-15 N2 -acetylimidazole. Using SABRE-SHEATH (Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei), transfer of spin order occurs from parahydrogen to acetylimidazole 15 N atoms and the acetyl 13 C site (≈263-fold enhancement), giving rise to relatively long hyperpolarization lifetimes at 0.3 T (T1 ≈52 s and ≈149 s for 13 C and 15 N, respectively). Immediately following polarization transfer, the 13 C-labeled acetyl group is hydrolytically cleaved to produce hyperpolarized 13 C-acetate/acetic acid (≈140-fold enhancement) and 15 N-imidazole (≈180-fold enhancement), the former with a 13 C T1 of ≈14 s at 0.3 T. Straightforward synthetic routes, efficient spin-relay of SABRE hyperpolarization, and facile bond cleavage open a door to the cheap and rapid generation of long-lived hyperpolarized states within a wide range of molecular targets, including biologically relevant carboxylic acid derivatives, for metabolic and pH imaging.

SABRE hyperpolarization enables high-sensitivity 1H and 13C benchtop NMR spectroscopy

Benchtop NMR spectrometers operating with low magnetic fields of 1-2 T at sub-ppm resolution show great promise as analytical platforms that can be used outside the traditional laboratory environment for industrial process monitoring. One current limitation that reduces the uptake of benchtop NMR is associated with the detection fields' reduced sensitivity. Here we demonstrate how para-hydrogen (p-H2) based signal amplification by reversible exchange (SABRE), a simple to achieve hyperpolarization technique, enhances agent detectability within the environment of a benchtop (1 T) NMR spectrometer so that informative 1H and 13C NMR spectra can be readily recorded for low-concentration analytes. SABRE-derived 1H NMR signal enhancements of up to 17 000-fold, corresponding to 1H polarization levels of P = 5.9%, were achieved for 26 mM pyridine in d4-methanol in a matter of seconds. Comparable enhancement levels can be achieved in both deuterated and protio solvents but now the SABRE-enhanced analyte signals dominate due to the comparatively weak thermally-polarized solvent response. The SABRE approach also enables the acquisition of 13C NMR spectra of analytes at natural isotopic abundance in a single scan as evidenced by hyperpolarized 13C NMR spectra of tens of millimolar concentrations of 4-methylpyridine. Now the associated signal enhancement factors are up to 45 500 fold (P = 4.0%) and achieved in just 15 s. Integration of an automated SABRE polarization system with the benchtop NMR spectrometer framework produces renewable and reproducible NMR signal enhancements that can be exploited for the collection of multi-dimensional NMR spectra, exemplified here by a SABRE-enhanced 2D COSY NMR spectrum.

Trace analysis in water-alcohol mixtures by continuous p-H2 hyperpolarization at high magnetic field

Nuclear magnetic resonance (NMR) studies of complex mixtures are often limited by the low sensitivity of the technique and by spectral overlap. We have recently reported on an NMR chemosensor on the basis of para-Hydrogen Induced Polarization that potentially addresses both these issues, albeit for specific classes of compounds. This approach makes use of Signal Amplification By Reversible Exchange (SABRE) catalysts in methanol and allows selective detection and quantification of dilute analytes in complex mixtures. Herein, we demonstrate that, despite a large decrease in attained hyperpolarization, this method can be extended to water-alcohol mixtures. Our approach was tested on whisky, where nitrogenous heterocyclic flavor components at low-micromolar concentration could be detected and quantified.

Complete magnetic field dependence of SABRE-derived polarization

Signal amplification by reversible exchange (SABRE) is a promising hyperpolarization technique, which makes use of spin-order transfer from parahydrogen (the H₂ molecule in its singlet spin state) to a to-be-polarized substrate in a transient organometallic complex, termed the SABRE complex. In this work, we present an experimental method for measuring the magnetic field dependence of the SABRE effect over an ultrawide field range, namely, from 10 nT to 10 T. This approach gives a way to determine the complete magnetic field dependence of SABRE-derived polarization. Here, we focus on SABRE polarization of spin-1/2 hetero-nuclei, such as ¹³ C and ¹⁵ N and measure their polarization in the entire accessible field range; experimental studies are supported by calculations of polarization. Features of the field dependence of polarization can be attributed to level anticrossings in the spin system of the SABRE complex. Features at magnetic fields of the order of 100 nT-1 μT correspond to "strong coupling" of protons and hetero-nuclei, whereas features found in the mT field range stem from "strong coupling" of the proton system. Our approach gives a way to measuring and analyzing the complete SABRE field dependence, to probing NMR parameters of SABRE complexes and to optimizing the polarization value.

Rapid Catalyst Capture Enables Metal-Free para-Hydrogen-Based Hyperpolarized Contrast Agents

Hyperpolarization techniques based on the use of para-hydrogen provide orders of magnitude signal enhancement for magnetic resonance spectroscopy and imaging. The main drawback limiting widespread applicability of para-hydrogen-based techniques in biomedicine is the presence of organometallic compounds (the polarization transfer catalysts) in solution with hyperpolarized contrast agents. These catalysts are typically complexes of platinum-group metals, and their administration in vivo should be avoided. Herein, we show how extraction of a hyperpolarized compound from an organic phase to an aqueous phase combined with a rapid (less than 10 s) Ir-based catalyst capture by metal scavenging agents can produce pure para-hydrogen-based hyperpolarized contrast agents, as demonstrated by high-resolution nuclear magnetic resonance (NMR) spectroscopy and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The presented methodology enables fast and efficient means of producing pure hyperpolarized aqueous solutions for biomedical and other uses.

Continuous hyperpolarization with parahydrogen in a membrane reactor

Hyperpolarization methods entail a high potential to boost the sensitivity of NMR. Even though the "Signal Amplification by Reversible Exchange" (SABRE) approach uses para-enriched hydrogen, p-H₂, to repeatedly achieve high polarization levels on target molecules without altering their chemical structure, such studies are often limited to batch experiments in NMR tubes. Alternatively, this work introduces a continuous flow setup including a membrane reactor for the p-H₂, supply and consecutive detection in a 1 T NMR spectrometer. Two SABRE substrates pyridine and nicotinamide were hyperpolarized, and more than 1000-fold signal enhancement was found. Our strategy combines low-field NMR spectrometry and a membrane flow reactor. This enables precise control of the experimental conditions such as liquid and gas pressures, and volume flow for ensuring repeatable maximum polarization.

Hyperpolarized NMR Spectroscopy: d-DNP, PHIP, and SABRE Techniques

The intensity of NMR signals can be enhanced by several orders of magnitude by using various techniques for the hyperpolarization of different molecules. Such approaches can overcome the main sensitivity challenges facing modern NMR/magnetic resonance imaging (MRI) techniques, whilst hyperpolarized fluids can also be used in a variety of applications in material science and biomedicine. This Focus Review considers the fundamentals of the preparation of hyperpolarized liquids and gases by using dissolution dynamic nuclear polarization (d-DNP) and parahydrogen-based techniques, such as signal amplification by reversible exchange (SABRE) and parahydrogen-induced polarization (PHIP), in both heterogeneous and homogeneous processes. The various new aspects in the formation and utilization of hyperpolarized fluids, along with the possibility of observing NMR signal enhancement, are described.

A leap forward in iridium-NHC catalysis: new horizons and mechanistic insights

This review summarises the most recent advances in Ir-NHC catalysis while revisiting all the classical reactions in which this type of catalyst has proved to be active. The influence of the ligand system and, in particular, the impact of the NHC ligand on the activity and selectivity of the reaction have been analysed, accompanied by an examination of the great variety of catalytic cycles hitherto reported. The reaction mechanisms so far proposed are described and commented on for each individual process. Moreover, some general considerations that attempt to explain the influence of the NHC from a mechanistic viewpoint are presented at the end of the review. The first sections are dedicated to the most widely explored reactions that use Ir-NHCs, i.e., hydrogenation and transfer hydrogenation, for which a general overview that tries to compile all the Ir-NHC catalysts hitherto reported for these processes is provided. The next sections deal with hydrogen borrowing, hydrosilylation, water splitting, dehydrogenation (of alcohols, alkanes, aminoboranes and formic acid), hydrogen isotope exchange (HIE), signal amplification by reversible exchange and C-H bond functionalisation (silylation and borylation). The last section compiles a series of reactions somewhat less explored for Ir-NHC catalysts that include the hydroalkynylation of imines, hydroamination, diboration of olefins, hydrolysis and methanolysis of silanes, arylation of aldehydes with boronic acids, addition of aroyl chlorides to alkynes, visible light driven reactions, isomerisation of alkenes, asymmetric intramolecular allylic amination and reactions that employ heterometallic catalysts containing at least one Ir-NHC unit.

Spin-Lattice Relaxation of Hyperpolarized Metronidazole in Signal Amplification by Reversible Exchange in Micro-Tesla Fields

Simultaneous reversible chemical exchange of parahydrogen and to-be-hyperpolarized substrate on metal centers enables spontaneous transfer of spin order from parahydrogen singlet to nuclear spins of the substrate. When performed at sub-micro-Tesla magnetic field, this technique of NMR Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH). SABRE-SHEATH has been shown to hyperpolarize nitrogen-15 sites of a wide range of biologically interesting molecules to a high polarization level (P > 20%) in one minute. Here, we report on a systematic study of 1H, 13C and 15N spin-lattice relaxation (T1) of metronidazole-13C2-15N2 in SABRE-SHEATH hyperpolarization process. In micro-Tesla range, we find that all 1H, 13C and 15N spins studied share approximately the same T1 values (ca. 4 s at the conditions studied) due to mixing of their Zeeman levels, which is consistent with the model of relayed SABRE-SHEATH effect. These T1 values are significantly lower than those at higher magnetic (i.e. the Earth's magnetic field and above), which exceed 3 minutes in some cases. Moreover, these relatively short T1 values observed below 1 micro-Tesla limit the polarization build-up process of SABRE-SHEATH- thereby, limiting maximum attainable 15N polarization. The relatively short nature of T1 values observed below 1 micro-Tesla is primarily caused by intermolecular interactions with quadrupolar iridium centers or dihydride protons of the employed polarization transfer catalyst, whereas intramolecular spin-spin interactions with 14N quadrupolar centers have significantly smaller contribution. The presented experimental results and their analysis will be beneficial for more rational design of SABRE-SHEATH (i) polarization transfer catalyst, and (ii) hyperpolarized molecular probes in the context of biomedical imaging and other applications.